To date, composite preparations comprising plural materials have been developed in order to attain unique characteristics that are not found in any single material. As a composite material, glass fiber-reinforced plastic had been widely used. Particularly, the development of carbon fibers and carbon fiber reinforced plastics (CFRP) has brought such composite materials into general use.
These composite materials have been widely used in sporting goods and so on, and have also gained much attention as light weight-, high intensity- and high elastic modulus-structural materials for aircrafts. In addition to the fiber-reinforced materials mentioned above, composite materials reinforced with fine particles have also been successfully developed. Composite materials, while generally regarded as structural materials for their structural properties such as strength and heat resistance, are increasingly being recognized as functional materials for their electrical, electronic, optical, and chemical characteristics.
As the prevalence of various electronic devices increases, problems such as malfunction of devices caused by static electricity and electromagnetic wave interference caused by noises from certain electronic components are also on the rise, thus creating an increased demand for materials that have excellent functional characteristics such as conductivities and damping abilities.
Traditional conductive polymer materials currently in wide use arc made by blending highly conductive fillers with low conductive polymers. In such materials, metallic fibers, metallic powders, carbon black, carbon fibers and other similar materials are generally used as conductive fillers. However, when using metallic fibers and metallic powders as the conductive filler, the materials thus obtained have poor corrosion resistance and mechanical strength. When using carbon fibers as the conductive filler, although a predetermined strength and elastic modulus may be obtained by adding relatively large amounts of the filler, electrical conductivity generally cannot be greatly enhanced by this approach. If one attempts to attain a predetermined conductivity by adding a large amount of filler, one would invariably degrade the intrinsic properties of the original polymer material. Incidentally, with respect to a carbon fiber, it is expected that the conductivity-imparting effect increases as its diameter becomes smaller at an equivalent additive amount, because the contact area between the fiber and the matrix polymer increases.
Carbon fibers may be manufactured by subjecting a precursor organic polymer, particularly, a continuous filament of cellulose or polyacrylonitrile, to thermal decomposition under a controlled condition, in which a forced tension on the precursor polymer is carefully maintained in order to achieve a good orientation of anisotropic sheets of carbon in the final product. In such manufacturing processes, the level of material loss during carbonization is high and the carbonization rate is slow. Therefore, carbon fibers made by these processes tend to be expensive.
In recent years, a different class of carbon fibers, known as urtrathin carbon fibers such as carbon nano structures, exemplified by the carbon nanotubes (hereinafter, referred to also as “CNT”), has become a focus of attention.
The graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered ring carbon networks, which bring about unique electrical properties, as well as chemical, mechanical, and thermal stabilities. As long as such urtrathin carbon fibers can retain such properties upon blending and dispersion in a solid material, including various resins, ceramics, metals, etc., or in liquid materials, including fuels, lubricant agents, etc., their usefulness as additives for improving material properties can be expected.
On the other hand, however, such fine carbon fibers unfortunately show an aggregate state even just after their synthesis. When these aggregates are used as-is, the fine carbon fibers would be poorly disperse, and thus the product obtained would not benefit from the desired properties of the nano structures. Accordingly, given a desired property such as electric conductivity for a matrix such as a resin, it is necessary that the fine carbon fibers would be added in a large amount.
Patent Literature 1 discloses a resin composition comprising aggregates wherein each of the aggregate is composed of mutually entangled carbon fibrils having 3.5-70 nit in diameter, and wherein the aggregates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. It is noted that the numeric data such as the maximum diameter, diameter, etc., for the carbon fibril aggregates are those measured prior to combining with a resin, as is clear from the descriptions in the examples and other parts of the Patent Literature 1.
Patent Literature 2 discloses a composite material where a carbon fibrous material is added to the matrix, the carbon fibrous material mainly comprising aggregates each of which is composed of carbon fibers having 50-5000 nm in diameter, the mutual contacting points among the carbon fibers being fixed with carbonized carbonaceous substance, and each aggregates having a size of 5 μm-500 μm. In the Patent Literature 2, the numeric data such as the size of aggregate, etc., are those measured prior to the combining into resin, too.
Using carbon fiber aggregates such as described above, it is expected that the dispersibility of carbon nano structures within a resin matrix will improve to a certain degree as compared to that of using bigger lumps of carbon fibers. The aggregates prepared by dispersing carbon fibrils under a certain shearing force, such as in a vibrating ball mill or the like according to the Patent Literature 1, however, have relatively high bulk densities. Thus, they do not fulfill the need for ideal additives that is capable of improving various characteristics, such as electric conductivity, of a matrix effectively at minuscule dosages.
The Patent Literature 2 discloses a carbon fibrous structure which is manufactured by heating carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression molding after synthesis of the carbon fibers, and wherein fixing of fibers at the contacting points is done by carbonization of organic residues primarily attached to the surface of the carbon fibers, or carbonization of an organic compound additionally added as a binder. Since fixing of carbon fibers is performed by such a heat treatment after synthesis of the carbon fibers, the affixing forces at the contacting points are weak and do not result in good electrical properties of the carbon fibrous structures. When these carbon fibrous structures are added to a matrix such as a resin, the carbon fibers fixed at the contacting points are easily detached from each other, and the carbon fibrous structures are no longer maintained in the matrix.
Incidentally, as the composite material described above, structural materials which needs high mechanical strength such as stiffness even granting that the formability is sacrificed to a certain extent, and materials which needs high electrical conductivity such as material for electrodes are involved in general. However, a large volume addition of the above mentioned carbon fibers or carbon fiber aggregates into the matrix, per se, is very difficult. Further, even if the products can be manufactured with such a high content of the carbon fibers or carbon fiber aggregates, the properties of the products thus obtained would be far from the intended ones.    [Patent Literature 1] Japanese patent No. 2862578    [Patent Literature 2] JP-2004-119386A